Visible light responsive photocatalysts can utilize visible light which accounts for large part of sunlight. The visible light responsive photocatalysts are expected to be applied for photodecomposition of organic substances and hydrogen production by splitting water. In particular, photocatalysts for the purpose of splitting water to produce hydrogen have received attention as photocatalysts usable for hydrogen generating process with the renewable energy. As a result, demand for highly active photocatalysts for splitting water has increased.
WO 2014/046305 A (PTL 1) discloses as an example a photocatalytic member comprising a base and a photocatalytic layer fixed thereon, the photocatalytic layer containing visible light responsive photocatalyst particles for hydrogen generation having a primary particle diameter of 100 nm or less; and visible light responsive photocatalyst particles for oxygen generation. In this example, the visible light responsive photocatalyst particles for hydrogen generation and the visible light responsive photocatalyst particles for oxygen generation are in contact with each other. According to the PTL 1, the photocatalytic member is reported to have a high hydrogen generation performance. On the other hand, further improvement for the hydrogen generation performance of the photocatalytic member has been demanded.
JP 2014-046236 A (PTL 2) discloses as an example semiconductor heteroparticles including first semiconductor particles bound to first metal particles and second semiconductor particles bound to second metal particles. In this example, the electric potential of the bottom end of the conduction band of the first semiconductor particle is negative, i.e., less noble, or closer to vacuum level than, relative to the electric potential of the upper end of the valence band of the second semiconductor particle, and furthermore, the work functions relative to a normal hydrogen electrode (NHE) of the first and second semiconductor particles are each independently equal to either the potential of the bottom end of the conduction band of the first semiconductor particle or the electric potential of the upper end of the valence band of the second semiconductor particle, or are each between these potentials. According to the PTL 2, the semiconductor heteroparticles are reported to show a photocatalytic function even without using an electron mediator. In the PTL 2, the photocatalytic activity of the semiconductor heteroparticles, when using a dispersion obtained by dispersing the heteroparticles in water, is specifically evaluated.
JP 2014-223629 A (PTL 3) discloses as an example an electrode which is formed by depositing photocatalyst particles on a base and is used for water photolysis reaction, wherein a semiconductor or good conductor which does not catalyze the reverse reaction of the water photolysis reaction is put between the photocatalyst particles as well as between the photocatalyst particles and the base. The semiconductor or good conductor can improve the electron conductivity of the photocatalytic layer to enhance the photoelectric conversion efficiency, and therefore, can increase the water photolysis reaction rate under light irradiation.
WO 2013/133338 A (PTL 4) discloses as an example an electrode for water photolysis reaction comprising a photocatalytic layer, a current collector layer and a contact layer which has a semiconductor or good conductor and is provided between the photocatalytic layer and the current collector layer. This electrode can increase conductive paths between the photocatalytic layer and the current collector layer without inhibiting light absorption by the photocatalyst.
JP 2012-187520 A (PTL 5) discloses as an example a photocatalyst-fixed product for splitting water, the photocatalyst-fixed product including a photocatalytic layer provided on a base, the photocatalytic layer including a visible light responsive photosemiconductor that is a nitride or oxynitride, a co-catalyst supported on the photosemiconductor, and a hydrophilic inorganic material.